The present invention relates generally to the field of resistors. In particular, the present invention relates to resistors that can be embedded within a dielectric material, such as in the manufacture of a printed circuit board.
A variety of thin film resistor structures, such as embeddable thin film resistors, are known. These resistor structures are typically prepared by the deposition of a resistive material on a conductive substrate, such as a copper foil. For example, International Patent Application WO 89/02212 (Rice) discloses the formation of resistive material by electroplating a layer of nickel-phosphorus on a copper foil. U.S. Pat. No. 6,208,234 B1 (Hunt et al.) discloses the deposition of a resistive material on a substrate using combustion chemical vapor deposition (xe2x80x9cCCVDxe2x80x9d) or controlled atmosphere CCVD (xe2x80x9cCACCVDxe2x80x9d). These materials are then typically trimmed to provide materials having the desired resistance.
The resistance of these resistor structures depends upon both the thickness of the resistive material and the resistivity of the resistive material. There is interest in the printed wiring board industry for resistors having higher resistivities. As the thickness of the resistive material decreases, the resistance of the material increases. Thus, the thickness of the resistive materials used in these resistor structures is decreasing. Thinner resistive material layers mean that the topography of the underlying conductive substrate becomes a significant consideration.
Copper foils are typically produced by the electrodeposition of copper from a solution onto a rotating drum. The side of the copper foil adjacent the drum is the smooth (or shiny) side while the other side has a much higher roughness (the matte side). The matte side of the foil typically provides better adhesion to a substrate, e.g. a polymeric layer such as a photoresist or prepreg. The shiny side of the foil must have some form of roughening layer deposited on it in order to provide sufficient adhesion to a polymeric coating or layer. A variety of roughening treatments for foils are known. For example, U.S. Pat. No. 5,679,230 (Fatcheric et al.) discloses copper foils having a roughening agent added to the shiny side and a nodular copper deposit on the matte side.
It is known to deposit a resistive material on the rough side of a copper foil in the formation of embeddable, thin film resistors. For example, in the above described International Patent Application WO 89/02212, the formation of a resistive structure by electroplating a layer of nickel-phosphorus as resistive material on the matte (or rough) side of a copper foil is described.
Applying the resistive material to the matte side of the foil is thought to improve the adhesion between the resistive material and the foil, as in the case of polymeric coatings. Thus, the rougher the surface, the better the adhesion of the resistive material to the foil. However, very rough surfaces cause other problems when thin resistive layers are used. As an example, a rougher foil surface requires more resistive material be deposited than a smoother foil surface to obtain a structure having a given sheet resistivity. Typically, the addition of more resistive material requires additional deposition procedures as compared to those required for a smoother foil. When the resistive material includes expensive metals, such as platinum, the cost using a rougher surface greatly increases, both from the increased cost due to more material being used as well as increased processing and/or manufacturing times.
The topographic structure of metal foils are conventionally evaluated by looking at the roughness of the surface, that is the peak-to-valley distance. Such roughness is variously reported as Rtm or Rms roughness of a surface. See, for example, U.S. Pat. No. 5,454,926 (Clouser et al.) which describes a copper foil having a matte side Rtm in the range of 4.5 to 18 xcexcm. However, another problem affecting thin film resistors is the directionality of the roughness of a copper foil. Drums for the electroplating of copper are polished to provide a smooth surface. Such polishing typically results in grooves, ridges or other surface imperfections on the drum which are circumferential. As a result, such imperfections are transferred from the drum to the copper foil during deposition. These imperfections create directionality in the copper foil, which is then transferred to the resistive material deposited thereon. As a result, thin film resistors typically have a different resistance in a first direction as compared to a direction orthogonal to it. Such difference in resistance may not be great, but it may cause defects in the finished electronic device if the resistor is installed in the wrong direction.
Accordingly, there is a need for metal foils suitable for use in thin film resistor manufacture wherein the foils have sufficient roughness for good adhesion of the resistive material and require less resistive material to obtain a given sheet resistivity than conventional metal foils. It is also desired that such metal foils have an isotropic roughness.
In one aspect, the present invention provides a structure including a conductive layer having first and second sides and a resistive material disposed on and in intimate contact with the first side of the conductive layer, wherein the first side of the conductive layer has an isotropic surface roughness having a Rz (din) value of 3 to 10 xcexcm and a peak-to-peak wavelength of 2 to 20 xcexcm.
In another aspect, the present invention provides a method of manufacturing a resistive structure including the steps of: a) providing a conductive layer have first and second sides wherein the first side has an isotropic surface roughness having a Rz (din) value of 3 to 10 xcexcm and a peak-to-peak wavelength of 2 to 20 xcexcm; and b) depositing a resistive material on the first side of the conductive later.
In yet another aspect, the present invention provides a printed wiring board having including a resistor wherein the resistor includes a conductive layer having first and second sides and a resistive material disposed on and in intimate contact with the first side of the conductive layer, wherein the first side of the conductive layer has an isotropic surface roughness having a Rz (din) value of 3 to 10 xcexcm and a peak-to-peak wavelength of 2 to 20 xcexcm.
The present invention also provides a conductive foil having a first side and a second side, wherein the first side has an isotropic roughness having a Rz (din) value of 3 to 10 xcexcm and a mean peak-to-peak wavelength of 2 to 20 xcexcm. Typically, the first side is the drum or smooth side of the foil.